Blame The Wicker Man. When the 1973 English film about Scottish islanders sacrificing a Christian policeman to their pagan gods emerged as a cult classic some 20 years later, it introduced a new generation to the strange and mystical world of folk music. "Before The Wicker Man, folk had come to be associated with real ale and geography teachers," says Johnny Trunk, who released the film's soundtrack on his Trunk label in 1995. "Then people began to realise how interesting this world was."

The weird folk revival has been building steadily ever since, but it is only now that is has boiled over, like a hearty mushroom broth, into the mainstream. Bert Jansch, one of the great British guitarists and a founder member of the 1960s folk/jazz/rock band Pentangle, is now a key influence on musicians including Adem, Joanna Newsom and Devendra Banhart, the latter of whom will be playing on Jansch's next album. Vashti Bunyan, whose 1970 album Just Another Diamond Day was a commercial disaster, has become a star for the first time at the age of 59.

"I can actually hear myself in the music of people like Devendra Banhart," says Jansch, who is headlining a concert at the Barbican that will also feature Bunyan and members of weird folk's young guard. "They're taking a different approach - I'm brought up in the songwriting tradition and they tend to make fragments of songs with no real beginning and end - but the connection is there."

Folk is a huge field, however, and the revival comes chiefly from the longhaired, fantastical side of the music. The forthcoming compilation Strange Folk (Albion Records) features a remarkable song by Barry Dransfield about a werewolf that cries tears of heartache as he rips off the clothes of a maiden, but interpretations of traditional songs by the socialist folk singer Ewan MacColl, a huge name in his day, would be out of place here. The pure strain of folk, in which one performs unaccompanied odes to loaves of bread, has remained the preserve of the traditional clubs.

"At Ewan MacColl's clubs in the 1960s you were expected to only sing songs from the area you came from," remembers Jansch, who horrified folk purists with Pentangle's jazz rhythm section and amplifiers. "But the whole thing was ridiculous. He passed himself off as a Scotsman when he was from Salford."

Notions of authenticity have dogged folk music ever since Bob Dylan going electric resulted in a mass shuffling of duffel coats in Britain's concert halls in 1965, despite the fact that traditional songs always get reinterpreted as they travel from village to village.

Now, as a generation of mostly urban types weave a fantasy of rural life from memories of childhood holidays and the last time they watched The Wicker Man, the spirit of ancient music is fused with the new once more.

· Folk Britannia begins on BBC4 on Fri 3 and is at the Barbican, Thu 2-Feb 4